Versatility of Electron Microscopy in Materials Science

Transcription

1 Hilfe2 Versatility of Electron Microscopy in Materials Science Exploring the micro-, nano- and atomic length scales Rolf Erni Electron Microscopy Center, EMPA

2 Electron Microscopy Center - EMPA Mission Launched: July 2009: to centralize the EM activities within EMPA Provide EM support (consulting & hands-on) for EMPA research groups Ceramics Thin films Surfaces & coatings Nanoparticles and nanostructures Wood? Provide (limited) EM services for industrial collaborators Collaborations with other EM centers

3 EMC EMPA Instruments 2 TEMs Philips CM30 Jeol JEM 2200FS (analytical STEM/TEM, 2009) 2 SEMs FEI XL30 ESEM FEI NovaNanoSEM (with EAWAG, 2009) Sample preparation Ion mill, plasma cleaner, polishing

4 Targets User training and guidance during the first steps. Support in the interpretation of data. Exit-plane wave reconstruction Simulations Tomographic reconstructions Towards analytical electron microscopy: spectroscopy, image analysis etc. Establish new techniques within EMPA: Electron energy-loss spectroscopy STEM, Z-contrast imaging Tomography

5 The Role of Electron Microscopy in Materials Science it gives better resolution than an optical microscope Whatever is being analyzed at some point one would like to image it. Almost half of all publications in materials science contain electron microscopy. It s multi-functional and easy-to-use. Atomic-resolution imaging. Atoms form solids. BUT It s in vacuum. Electrons can damage. H. Rose (1994)

6 The Pioneers Electromagnetic fields act as lenses. && r = rf ( z) Hamiltonian analogy. The movement of particles can be described in optical terms Hans Busch 1926 Electrons have a wavelength. λ = h p 1924 Louis de Broglie

7 Aberration-corrected microscopes: 1997/ /32 Max Knoll & Ernst Ruska 1950 Übermikroskop 2010

8 Types of Electron Microscopes Transmission Electron Microscope TEM Scanning Electron Microscope SEM Scanning Transmission Electron Microscope (STEM) Von Ardenne (1938) Low-Energy Electron Microscope LEEM Spin Polarized Low-Energy Electron Microscope SPLEEM

9 (Scanning) Transmission Electron Microscopy Transmission = Thin, electron-transparent specimens are needed: nm.

10 The Chicken Problem Otto Scherzer: Die Strahlenschädigung der Objekte als Grenze für die hochauflösende Elektronen-Mikroskopie. Berichte der Bunsen-Gesellschaft 74 (1970) The Object The Specimen The Image What we expect. What we don t (want to) expect.

11 Typical STEM/TEM Techniques Imaging & Diffraction Conventional TEM: dislocations, grains, particles High-Resolution TEM: atomic structure Z-contrast imaging in STEM Electron diffraction Bright field imaging (Weak-beam) Dark-field imaging

12 Typical STEM/TEM Techniques Imaging & Diffraction Conventional TEM: dislocations, grains, particles High-Resolution TEM: atomic structure Z-contrast imaging in STEM Electron diffraction HRTEM Phase contrast transfer function

13 Typical STEM/TEM Techniques Imaging & Diffraction Conventional TEM: dislocations, grains, particles High-Resolution TEM: atomic structure Z-contrast imaging in STEM Electron diffraction Z-contrast STEM 200 nm

14 Typical STEM/TEM Techniques Imaging & Diffraction Conventional TEM: dislocations, grains, particles High-Resolution TEM: atomic structure Z-contrast imaging in STEM Electron diffraction Electron diffraction

15 Typical STEM/TEM Techniques Analytical techniques Energy-dispersive X-ray spectroscopy: EDS: (quantitative) elemental analysis Electron energy-loss spectroscopy: EELS: elements, bonding, electronic state, valence Energy-filtered TEM: elemental distribution maps EDS

16 Typical STEM/TEM Techniques Analytical techniques Energy-dispersive X-ray spectroscopy: EDS: (quantitative) elemental analysis Electron energy-loss spectroscopy: EELS: elements, bonding, electronic state, valence Energy-filtered TEM: elemental distribution maps EELS Oxygen K-edge

17 Typical STEM/TEM Techniques Analytical techniques Energy-dispersive X-ray spectroscopy: EDS: (quantitative) elemental analysis Electron energy-loss spectroscopy: EELS: elements, bonding, electronic state, valence Energy-filtered TEM: elemental distribution maps Element-specific imaging EFTEM Multi-Layer Sample: Blue: Ti-rich Red: La-rich

18 Beyond Typical STEM/TEM Techniques In-situ TEM Environmental TEM: gas atmosphere (catalysis, synthesis). Mechanical testing or electronic measurements. Lorentz Microscopy Magnetic samples are being imaged in a field-free space: magnetic domains. Holography: off-axis, on-axis Analytics Restoration of the complex electron wave solving the phase problem. Valence electron energy-loss spectroscopy: dielectric and optical information, plasmonics Cathodoluminescence Dynamical TEM Electron pulses: nanosecond resolution (maybe even better ).

19 Resolution in Electron Microscopy Scherzer theorem For stationary rotationally symmetric electromagnetic lenses which are free of space charges, C 3 and C C are always finite positive. Zeitschrift für Physik 101 (1936) 593. Spherical Aberration C 3 Chromatic Aberration C C The resolution in conventional electron microscopy is limited by the spherical aberration. Round lenses also suffer from coma, field astigmatism, field curvature and distortion, but these off-axial aberrations, all vanish for object points on the optical axis and are thus not of highest importance for atomic-resolution microscopy.

20 It s a practical but not a fundamental limitation O. Scherzer, Robert Seeliger, Gottfried Möllenstedt ( ) Archard, Hans Deltrap ( ) Krivanek, Dellby (1997, STEM), Zach, Haider (1995, SEM) Quadrupole-Octupole Correctors Beck, Albert Crewe ( ) Rose, Haider (1998): Hexapole Corrector (TEM)

21 Solving the Resolution Problem: New Possibilities Conventional Electron Microscopy The resolution of conventional microscopes is determined by Spherical aberration Wavelength (2-4 pm) Conventional high resolution microscopes are operated between 200 and 1200 kev. Many (nano-)materials end up like Conventional Aberration corrected Aberration-Corrected Electron Microscopy The wavelength (high tension) is not critical anymore. The electron energy can be adjusted according to the requirements of the specimen: staying in the frying regime. Plus: better contrast, higher sensitivity, less artifacts

22 Where are the limits? Single atoms? Graphene: a single atomic layer of carbon atoms. 80 kv, aberration corrected TEM Simplified transfer function direct image interpretation Single carbon atoms can be observed.

23 Graphene at the Edge: Dynamics Electrons activate atoms at the edge of graphene. Armchair or Zig-zag? 1 frame per second Zig-zag edges are more stable The stability of belts of graphene depends on the direction the hexagons are arranged. New high-resolution imaging techniques can provide information about the dynamics of individual atoms at room temperature.

24 Other Edge configurations Armchair or Zig-zag? Or Zig-zag configuration might be metastable: Reconstructed zig-zag: Rec-zag configuration needs to be considered too. An observation is triggered by an expectation.

25 Dynamics of Lattice Defects in Graphene Appearance and disappearance of a Stone-Wales defect Electron dose: 44.4 A/cm 2 (~28000 electrons/sec Å 2 ) 0 sec 4 sec Disappearance of a vacancy Pentagons and heptagons 0 sec 4 sec Electron irradiation provides activation energy for defect formation. Theoretically predicted lattice defects can be confirmed. Defect formation occurs on a time-scale that can be monitored in a TEM. J. C. Meyer et al., Nano Letters 8 (2008) 3582.

26 Nanoparticles: Building Blocks of Meta-Materials Using electrons to probe dielectric behavior of materials Opal Opal in a SEM Special properties A special material? 2 μm No special material - just silica spheres! It s glass but it doesn t look like! A simple material built of individual building blocks can lead to unique properties. Knowing the behavior of a single building block and how they interact, enables the design of materials with engineered (dielectric) properties. - Negative refraction - Special optical lenses - Cloaking devices - Surface coatings Requires a high energy resolution EELS: preferably an electron monochromator.

27 Surface Plasmons on Gold Nanorods Plasmon modes of a single nano-rod of gold. Low-loss spectroscopy directly reveals the longitudinal and transverse plasmon modes. Monochromated STEM/VEELS measurements: 80 kv, energy resolution 90 mev, raw data. HAADF STEM signal Plasmon-mode mapping Longitudinal mode ΔE= ev Longitudinal Transverse mode Transverse mode ΔE= ev RGB map Blue: HAADF Green: L-mode Red: T-mode

28 Particle Interaction: field enhancement Triangle of Au rods Transversal peak (2.5 ev) is enhanced near left corner. Longitudinal peak (1.7 ev) is enhanced at the two other corners. Center of the triangle behaves like bulk material. Plasmon mode: ev 1 Plasmon mode: ev 2 3 Local field enhancement known from optical measurements can be identified and characterized. Designing metamaterials requires knowledge about the spatial distribution of the dielectric properties of nanoparticles.

29 Optical Spectroscopy with Electrons Combining the spatial resolution of electron microscopy with high resolution electron energy-loss spectroscopy allows for locally probing the band structure of materials. Low-loss or Valence Electron Energy-Loss Spectroscopy An electron monochromator enables highest energy resolution. Band-gap variation in In-rich InGaN quantum wells; In-rich agglomerates can act as quantum dots. GaN matrix: 3.3±0.05 ev InN QW: 2.9±0.32 ev Identification of the 2175 Å absorption feature in Interplanetary Dust Particles

30 The Projection Problem: Tomography A (S)TEM provides a projection of an (3D) object. A single projection can be misleading. From a series of projection, the 3D structure of an object can be reconstructed. Image capture Back projection For a perfect reconstruction, an infinite number of images are required: artifacts are unavoidable! From: Science of microscopy, Part I, p. 540.

31 From 2D to 3D Test-Objects: Agglomerate of nanoparticles. 100 nm Highest resolution feasible in (conventional) electron tomography: ~2-3 nm 3 Where did the resolution go?

32 Summary Electron microscopy is more than imaging and diffraction. Novel imaging techniques allow for observing the motion of individual atoms. Analytical techniques benefit from the resolution feasible in electron microscopes. Electron microscopy can provide a rather comprehensive picture about novel materials and this information can be combined with other techniques.

33 Acknowledgment Graphene Alex Zettl, Nasim Alem, Will Garret, Çağlar Girit (UC Berekeley) Marta Rossell (ETHZ) Plasmons Mouss N gom, Ted Norris (University of Michigan) Tomography Daniel Schreier, Magdalena Parlinska (Empa) Sandra Van Aert, Joost Batenburg (U Antwerp), Marta Rossell (ETHZ) National Center for Electron Microscopy, Lawrence Berkeley Lab.

34 Literature Graphene J. C. Meyer, C. Kisielowski, R. Erni, M. D. Rossell, M. F. Crommie, A. Zettl, Direct imaging of lattice atoms and topological defects in graphene membranes, Nano Letters 8 (2008) Ç. Ö. Girit, J. C. Meyer, R. Erni, M. D. Rossell, C. Kisielowski, L. Yang, C.-H. Park, M. F. Crommie, M. L. Cohen, S. G. Louie, A. Zettl, Graphene at the edge: stability and dynamics, Science 323 (2009) Science 323 (2009) Plasmons & VEELS R. Erni and N. D. Browning, Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy, Ultramicroscopy 104 (2005) M. N Gom, S. Li, G. Schatz, R. Erni, A. Agarwal, N. Kotov, T. B. Norris, Electron beam mapping of plasmon resonances in electromagnetically interacting gold nanorods, Physical Review B 80 (2009) Transmission Electron Microscopy R. Erni, N. D. Browning, Transmission Electron Microscopy, Encyclopedia of Chemical Processing, R. M. Goodman, J. W. Steed, eds. (Taylor & Francis, Dekker, Inc., New York, 2006), Aberration-Corrected Electron Microscopy R. Erni, Aberration-corrected imaging in transmission electron microscopy: an introduction, Imperial College Press, 340p., ISBN-10: (expected July 2010).